HPSF THUMBNAIL HOW-TO

The VT_CF Format

Thumbnail information is stored as a VT_CF, or Thumbnail Variant. The
Thumbnail Variant is used to store various types of information in a
clipboard. The VT_CF can store information in formats for the Macintosh or
Windows clipboard.

There are many types of data that can be copied to the clipboard, but the
only types of information needed for thumbnail manipulation are the image
formats.